1. Field of the Invention
The present invention relates to optically butted electro-optical components in general, and to optically butted CCD (charge coupled devices) linear array sensors in image reading apparatus for providing electric signals corresponding to the image of an original by optical scanning, in particular.
2. Description of the Prior Art
In an image reading apparatus employed, for example, in a facsimile apparatus or a graphic scanner, an original moving in a determined direction is illuminated with light, and the reflected or transmitted light is focused through a lens onto a photoelectric converter, such as a linear photosensor array, to provide time-sequential electric image signals corresponding to a scanning line. Said photosensor array is usually composed of a CCD array or a photodiode array on which the image of a scanning line is focused by said lens.
The resolving power of the image is determined by that of the photosensor array, which is equal to the number of photoelectric converting elements in said photosensor array. Usually the number of said-elements in a today's CCD photosensor array is limited to 7500. CCD sensors with 10.000 components do exist but they are disproportionately expensive.
Image-reading apparatus are known comprising optical path dividing means, e.g. semitransparent prisms, for dividing the path for the light beam of a scanning line into two paths that each lead to a subfocal plane. A number of CCD's are mounted in both focal subplanes in staggered relationship so as to form optically butted CCD's. This technique is disclosed, e.g. in the article "A high-resolution, high-speed film scanner system using optically butted charge-coupled devices", by R. A. Whittlesey in SPIE, Vol. 149, 1978, p. 156-185.
A problem with the described arrangement is the exact spacial mounting of each CCD in its corresponding focal sub-planes. This is generally done by means of adjustment screws, levers, springs and the like that have their effect on five parameters that determine the exact position of a CCD component, viz. the lateral and the vertical position of the component, the rotation in its own plane, the axial position on the optical axis and the tilting about an axis normal to the optical axis and its own longitudinal axis. The process of adjustment is complicated by the fact that a CCD chip is often mounted inaccurately in its housing by the manufacturer. For instance, tolerances up to 0.5 mm between the positions of the lateral ends of a chip and the rear or front wall of the housing, usually a ceramic housing covered and closed by a glass panel, are not uncommon.